Solventless nontoxic high refraction index and low birefringence organic/inorganic hybrid materials

ABSTRACT

Enclosed are high refractive index and low birefringence organic/inorganic hybrid materials useful for optical applications. They are prepared from solventless metal aliphatic acryl alkoxides. The metal acryl alkoxides are synthesized from exchanging acryl alcohol with metal alkoxides, and are hydrolyzed into metal oxide nanoparticles and are well dispersed in the acrylate matrix. Then they are polymerized into organic/inorganic hybrid materials containing metal oxide in polyacrylate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high refractive index and lowbirefringence organic/inorganic hybrid materials useful for opticalapplications.

2. Description of the Related Art

Many optical applications require high refractive index materialsincluding filters, transmitters, reflectors, lenses, optical waveguides,sensors, devices, adhesives for optical components and device assembly,index matched materials for solid state laser, optical components.

The organic/inorganic hybrid materials have been used to prepare highrefractive index materials. For examples, Wilkes et al. (U.S. Pat. No.5,109,080) have been reported that a refractive index ranged from1.60-1.76 can be obtained from a solvent based polyarylether(sulfone)containing SiO₂—TiO₂—ZrO₂. It is known that the aromatic moiety ofpolymer exhibits undesired birefringence properties for opticalapplications (Chem. & Eng. News p14-15, Dec. 20, 1999). The system alsocontained tetrahydrofuran and alcohol solvents. The tetrahydrofuran wasused to dissolve polyarylether(sulfone) and the alcohol was used toobtain the metal oxide precursor solution. The solvents were also used(1) to carry out the reaction between organic component and inorganiccomponent, and (2) to adjust the viscosity of the system for the ease ofprocessing. However, the solvents create pollution problems during theprocessing. Zimmerman et al. (L. Zimmermann, M. Weibel, W. Caseri and U.W. Suter, J. Mater. Res., 8(7), 1993, p. 1742-48) prepared a gelatin-PbSsystem that has a refractive index range from 1.5 to 2.5.Kyprianidou-Leodidou et al (T. Kyprianidou-Leodidou, H-J. Althaus, Y.Wyser, D. Vetter, M. Bucher, W. Caseri and U. W. Suter, J. Mater. Res.12(8), 1997, p. 2198-2206) synthesized high refractive index materialfrom polyethyleneoxide-FeS (refractive index 2.5-2.8) andpolyethyleneoxide-PbS (refractive index 3.9). However, gelatin andpolyethyleneoxide are hydrophilic and absorb moisture easily, lackmechanical strength and dimensional stability. PbS is undesirable in theapplication due to its highly toxic nature of lead compound. Therefore,there is a need for an environmental friendly high refractive index. lowbirefringence and nontoxic organic/inorganic hybrid materials.

SUMMARY OF THE INVENTION

It is an object of the present application to provide high refractiveindex and low birefringence organic/inorganic hybrid materials areprepared by solventless metal aliphatic acryl alkoxides. The metalaliphatic acryl alkoxides are synthesized first. Then, the metalaliphatic acryl alkoxides are underwent hydrolysis to form nanoparticlemetal oxides dispersed acrylates. They can be free radical polymerized(via thermal or photo) into high refractive index, low birefringence,high mechanical strength and low moisture absorption metal oxidedispersed aliphatic polyacrylates.

It is a further object of the present invention to provideorganic/inorganic hybrid materials that are solventless, nontoxic, highrefractive index, low birefringence, high mechanical strength and lowmoisture absorption.

It is an additional object of the present invention to provide a methodof making organic/inorganic materials of present invention.

A more complete understanding of these and other features and advantagesof the present invention will become apparent from a carefulconsideration of the following detailed description of certainembodiments and the illustrations as shown in the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a thermogravimetric analysis of organic/inorganic hybridmaterials;

FIG. 2 is a UV-VIS spectra of organic/inorganic hybrid materials;

FIG. 3 is a TEM photo of TiO₂/polyacrylate;

FIG. 4 is a TEM photo of TiO₂/Bi₂O₃/polyacrylate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Solventless high refractive index and low birefringenceorganic/inorganic hybrid materials are prepared by three steps. Thefirst step is to synthesize solventless metal aliphatic acryl alkoxides.The second step is to hydrolyze the metal aliphatic acryl alkoxides andform nanoparticle metal oxides dispersed acrylates. The third step is tofree radical polymerize (via thermal or photo) the acrylates by freeradical polymerization into high refractive index, low birefringence,high mechanical strength and low moisture absorption metal oxidedispersed aliphatic polyacrylates.

In the first step, the solventless metal aliphatic acryl alkoxides aresynthesized. They have the general formula ofM[—OR₁—O—CO—C(R₂)═CR₃R₄]_(n). Where the M is a metal element or amixture of metal elements. The metal can be selected from the metals inthe periodic table except toxic metal such as lead. The metals with theatomic number greater than the silicon element is preferred. The n valueis dependent on the valence of metal. Where R1 is a straight chain alkylgroup or branched alkyl group. The straight chain is preferred with theformula of (—CH2—)n. Where n is equal to 1 to 12 and n is equal to 1 to4 is preferred. Where R2, R3, R4 can be a hydrogen atom or straightchain alkyl group (—CH2—)n or branched alkyl group. The straight chainalkyl group is preferred, n is equal to 1 to 12 and n is equal to 1 to 4is preferred. The metal aliphatic acryl alkoxides are synthesized eitherby reacting metal with acrylate alcohol or by reacting metal alkoxidewith acrylate alcohol through an alcohol exchange. The alcohol exchangeis preferred, so a wide range of metal type can be selected. The alcoholexchange reaction is shown in the equation (1):

M(—OR)_(n)+HO—R₁—O—CO—C(R₂)═CR₃R₄M[—OR₁—O—CO—C(R₂)═CR₃R₄]_(n)+ROH  (1)

Where R is a straight chain alkyl group or branched alkyl group. Thestraight chain is preferred with the formula of (—CH₂—)_(n). Where n isequal to 1 to 12 and n is equal to 1 to 4 is preferred. The reaction canbe carried out in a moderate temperature such as 50 to 100° C. with orwithout catalysts. The catalysts are acids or bases. They can beinorganic catalysts or organic catalysts. The inorganic acid catalystsinclude HF, HBr, HCl, HNO₃ and H₂SO₄, and other mineral acids. Theorganic acid catalysts include aliphatic or aromatic carboxylic acids,etc. The inorganic bases include NH₄OH, NaOH, KOH, etc. The organicbases include aliphatic or aromatic amines, etc. The preferred reactionconditions are 60° C. reflux without catalyst under the absolute dry andinert gas conditions (˜0% relative humidity) such as dry argon or drynitrogen. The nitrogen is preferred due to its low cost. The byproductROH was removed by a vacuum distillation.

In the second step, the metal acryl alkoxides are underwent acid or basecatalyzed hydrolysis to form nanoparticle metal oxides dispersedacrylate monomers. The hydrolysis reaction is shown in the equation (2).

The equation (2) is a hydrolysis reaction, which is carried out in theacid, or base catalyzed condition to form metal oxide nanoparticles insuit in the acrylate monomer at room temperature. The catalysts can beinorganic or organic based catalysts as mentioned in the reaction (1).The formation of metal oxide particles is through the chemical reaction,so they can be nano size and dispersed evenly in the acrylate matrix.The nano size of particle is important, so the material can betransparent in visible light with high light transmission.

In the third step, the acrylate monomers are polymerized intopolyacrylates via thermal or photo initiated free radical polymerizationaccording to equation (3):

The polymerization can be carried out either by thermal induced or photoinduced radical polymerization. For thermal induced polymerization, athermal initiator or a mixture of thermal initiators is required.Peroxides, disulfides, diazo compounds are well known to those skilledin the art of thermal initiators. The diazo compounds of2,2′-azobisisobutyronitrile (AIBN) is preferred. The weight percentageof 0.1-1.0 of thermal initiator can be used. The reaction is carried outin a moderate temperature ranged from 50 to 100° C. for several hours.For photo induced polymerization, a photo initiator or a mixture ofphoto initiators is required. Benzoin ether derivatives, benzophenonederivatives, acetophenone derivatives are well known to those skilled inthe art of photo initiators. 2,2-dimethoxy-2-phenyl acetyl phenone, soldas Irg 651 by Ciba-Geigy is preferred. The weight percentage of 0.5-5 ofphoto initiator can be used. A UV light source is used to carry out thereaction from several seconds to several minutes. For a thin film sample(<1 mm thickness), the photo initiation is preferred. The thin film isprepared by a spin coating method. For a bulk sample (>1 mm thickness),the thermal initiation is preferred for a through cure of thick area.The bulk material is prepared by casting in a mold method. Therefractive index of the material is measured by an Ellipsometer (GaerTner Model 116C) in the visible range (400-1000 nm). The opticaltransmission of the materials is measured by a UV-Vis spectrophotometer(Jasco H-7100))

The following examples further illustrate this invention:

EXAMPLE 1 Preparation of Titanium Methacryl Ethoxide

In a dry nitrogen glove box, 100 ml of dry tetrahydrofuran, 10.33 gramsof titanium n-propoxide were placed in a 250 ml flask and mixed themwell at room temperature. The 18.73 grams of hydroxy ethyl methacrylatewere added into the mixture and reacted at room temperature. As soon asthe solution became golden yellow, a vacuum distillation was used toremove propanol by product. The distillation was continued for 12 hoursto remove tetrahydrofuran and propanol completely. A solventlesstitanium methacryl ethoxide was obtained. The structure of the productwas identified by NMR and IR.

EXAMPLE 2 Preparation of Titanium Bismuth Methacryl Ethoxide

In a dry nitrogen glove box, 1.5 grams of titanium methacrylate ethoxideprepared from example 1, 8.16 grams of hydroxy ethyl methacrylate and2-3 drops of water were mixed for 10 min in a flask. The 13.5 grams of1.7×10⁻⁴ mole of bismuth methoxy ethoxide in tetrahydrofuran was addedinto the mixture and reflux at 60° C. for one hour. A vacuumdistillation was used to remove tetrahydrofuran and methoxy ethanol byproduct at room temperature for 8 hours. A solventless titanium bismuthmethacryl ethoxide was obtained. The structure of the product wasidentified by NMR and IR.

EXAMPLE 3 Preparation of Film Sample

An about 20 grams of 10% (vol.) aqueous hydroxy ethyl methacrylate wasadded into a mixture of 100 grams (96 gram of metal methacryl ethoxideand 4 gram of 2,2-dimethoxy-2-phenyl acetophenone (photoinitiator)) tohydrolyze the mixture. The exact amount of water used in the hydrolysiswas dependent on the gel rate of the mixture. The mixture was hydrolyzedfor 2 minutes, and then the hydrolyzed mixture was spin coated on a Siwafer substrate. The sample was irradiated with an UV light (UVP Co.Model UVGL-25) at 254 nm for 3 min. and 365 nm for 3 min to cure thefilm.

EXAMPLE 4 Preparation of Monolith Sample

The photoinitiator of above mixture was replaced by a 0.5 wt % of2,2′-azobis (isobutyronitrile) thermal initiator. The mixture was pouredinto a mold with a dimension of 1 cm in diameter and 3 mm in thickness.The hydrolysis time was increased from the above film sample time 2minutes to 30 minutes, then the hydrolyzed sample was cured at 50° C./6hrs, 60° C./24 hrs and 90° C./48 hrs.

EXAMPLE 5 Characterization of Organic/Inorganic Hybrid Materials

a. Refractive Index Measurement

The refractive index of the cured film was measured by an Ellipsometer(Gaer Tner Co.). The results are shown in Table 1

TABLE 1 Refractive Index of Organic/Inorganic Hybrid Materials. MaterialRefractive Index Polyhydroxyethylmethacrylate-TiO₂ 1.61Polyhydroxyethylmethacrylate-TiO₂/Bi₂O₃ 1.69Polyhydroxyethylmethacrylate 1.51

The refractive index of the neat polymer: polyhydroxyethylmethacrylatehas been increased from 1.51 to 1.61 for TiO₂ dispersed polymer and to1.69 for TiO₂/Bi₂O₃ dispersed polymer.

b. Thermogravimetric Analysis

The sample was analyzed at 5° C./min from room temperature to 800° C. innitrogen using DuPont 9900 TGA 954 instrument. The results are shown inFIG. 1. The hybrid materials have shown an improved thermal stability;for a 10% weight loss temperature, the hybrid materials have a higherdecomposition temperature (325° C.) than that of (250° C.) neat acrylatepolymer.

c. The Optical Transmission Spectrum Analysis

The UV-VIS transmission spectra were measured by Jasco H-7100. Theresults are shown in FIG. 2. Both samples exhibited more than 95%transmission in the visible region.

d. Transmission Electron Microscope (TEM) Analysis

Hitachi H-7100 instrument was used to analyze the samples. The resultsare shown in FIG. 3 and FIG. 4. Both of the materials are shown themetal oxides are well dispersed in the polyacrylate matrix. The size ofthe metal oxides is in the range of 10-20 nm, which is smaller than thesize of visible light, so the samples are transparent.

Whereas particular embodiments of the invention have been describedabove for purposes of illustration, it will be appreciated by thoseskilled in the art that numerous variations of the details may be madewithout departing from the invention as described in the appendedclaims.

What we claim is:
 1. A compound metal aliphatic acryl alkoxide havingthe general formula of M[—OR₁—O—CO—C(R₂)═CR₃R₄]_(n) wherein saidcompound is prepared for exchanging acryl alcohol HO—R₁—O—CO—CR₂═CR₃R₄with metal alkoxide M[—O—(R)—(O)a—(R)_(b)—CH₃]_(n), and wherein M is ametal element or a mixture of metal elements, R₁ is an alkyl group,R₂R₃R₄ is a hydrogen atom or an alkyl group, a is equal to 0 or 1, b isequal to 0 to 12, and n is equal to 1 to
 12. 2. A compound as claimed inclaim 1 wherein R₁ is a straight chain alkyl group or branched alkylgroup; the straight chain is preferred with the formula of (—CH₂—)_(n),where n is preferably equal to 1 to 4 and R₂, R₃, R₄ can be a hydrogenatom or straight chain alkyl group (—CH₂—)_(n) or branched alkyl group.3. A compound as claimed in claim 1, wherein the acryl alcoholHO—R₁—O—CO—C(R₂)═CR₃R₄ is HO—CH₂CH₂—O—CO—C(CH₃)═CH₂.
 4. A compound asclaimed in claim 1, wherein in metal alkoxideM[—O—(R)—(O)a—(R)_(b)—CH₃]_(n), the metal M is selected from the metalsin the periodic table except toxic metal such as lead; the metals withthe atomic number greater than the silicon element are preferred such astitanium or bismuth; while R is a straight chain alkyl groups orbranched alkyl groups; the straight chain is preferred with the formulaof —(CH₂—)_(x), where x is preferably equal to 1 to 4, and b ispreferably equal to 0 to
 4. 5. A compound as claimed in claim 1, whereinthe metal alkoxide M[—O—(R)—(O)a—(R)_(b)—CH₃]_(n) is Ti[—O—(CH₂)₃CH₃]₄,or Bi[—O—(CH₂)₂—OCH₃]₃.
 6. A compound metal aliphatic acryl alkoxideobtained from the alcohol exchanging reaction as claimed in claim 1,wherein said compound is Ti[—OCH₂CH₂—O—CO—C(CH₃)═CH₂]₄.
 7. A compoundmetal aliphatic acryl alkoxide obtained from the alcohol exchangingreaction as claimed in claim wherein said compound isBi[—OCH₂CH₂—O—CO—C(CH₃)═CH₂]₃.
 8. A compound metal aliphatic acrylalkoxide obtained from the alcohol exchanging reaction as claimed inclaim 1, wherein said compound is a mixture ofTi[—OCH₂CH₂—O—CO—C(CH₃)═CH₂]₄ and Bi[—O—CH₂CH₂—O—CO—C(CH₃)═CH₂]₃ withthe composition of{Ti(—O—CH₂CH₂—O—CO—C(CH₃)═CH₂)₄}_(1.5){Bi[—OCH₂CH₂—O—CO—C (CH₃)═CH₂]₃}₁.